Pressure effect on optical properties of modified Gaussian quantum dots

In the present work, we intend to study the pressure effect on optical properties of spherical quantum dots by using the modified Gaussian potential. In this regard, the linear, nonlinear and total intersubband absorption coefficients and refractive index changes are investigated for different hydrostatic pressures. According to the results obtained from the present work, it is deduced that: (i) the linear, nonlinear and total refractive index changes decrease and shift towards higher energies when the pressure increases and (ii) the linear, nonlinear and total absorption coefficients increase and shift towards higher energies by increasing the pressure.     

Intense laser field effects on the linear and nonlinear intersubband optical properties of a semi-parabolic quantum well

By using the compact-density matrix approach, the effect of a nonresonant intense laser field on the linear and nonlinear optical absorptions based on intersubband transitions and the refractive index changes in an asymmetric semiconductor quantum well have been presented. Our results show that the peak position of the absorption coefficient is sensitive to intense laser field, the absorption maximum shifts towards lower energies for increasing intense laser field value. Also we observe as the intense laser field strength increases, the total refractive index change has been increased in magnitude and also shifted towards lower energies. The results indicate that linear and nonlinear optical properties of the low dimensional semiconductor heterostructures can be adjusted in a desired energy range by using intense laser field.     

Intersubband optical absorption coefficients and refractive index changes in modulation-doped asymmetric double quantum well

The linear and the third-order nonlinear optical absorption coefficients and refractive index changes in a modulation-doped asymmetric double quantum well are studied theoretically. The electron energy levels and the envelope wave functions in this structure are calculated by the Schrödinger and Poisson equations self-consistently in the effective mass approximation. The analytical expressions of optical properties are obtained by using the compact density-matrix approach. In this regard, the linear, nonlinear and total intersubband absorption coefficients and refractive index changes are investigated as a function of right-well width (Lw₂) of asymmetric double quantum well. Our results show that the total absorption coefficients and refractive index changes shift toward higher energies as the right-well width decreases. In addition, the total optical absorption coefficients and refractive index changes is strongly dependent on the incident optical intensity.     

Linear and nonlinear optical properties in spherical quantum dots: Rosen-Morse potential

In this paper, we have studied optical properties of spherical quantum dots by using Rosen-Morse potential. In this regard, we have first solved the Schrodinger equation and obtained energy levels and wave functions by applying Nikiforov-Uvarov (NU) method. Then, by using density matrix method, we have applied analytical expressions for the linear and third-order nonlinear absorption coefficient and refractive index changes. The results show that the total refractive index changes and the absorption coefficients increase and shift towards higher by enhancing height potential.     

Excited-state absorptions of an exciton confined in a quantum dot

The optical absorptions of an exciton with the higher excited states in a disc-like quantum dot are investigated. Calculations are made by using the method of numerical diagonalization of Hamiltonian matrix within the effective-mass approximation. With typical semiconducting GaAs based materials, the linear, third-order nonlinear, total optical absorption coefficients and refractive index changes have been calculated for the s–p, p–d, and d–f transitions. The results show that as the angular momentum quantum number of transitions increases, the absorption peaks shift towards lower energies and the absorption intensities increase.     

Electron-related nonlinear optical properties of cylindrical quantum dot with the Rosen-Morse axial potential

We present a theoretical study on the effects of intense laser field(ILF) and static electric field on the linear and nonlinear optical properties of a cylindrical quantum dot with Rosen-Morse axial potential under the framework of effective mass and parabolic band approximations. This study also takes into account the effects of the structure parameters(η, V_1, and R). The analytical expressions of the linear, third-order nonlinear and total optical absorption coefficients(TOACs)and the relative refractive index changes(RRICs) are obtained by using the compact-densitymatrix approach. The results of numerical calculations show that the resonant peak position of the TOACs and RRICs shifts towards lower energies and the magnitude of the peak increases with the effect of the static electric field and ILF. In addition, it is observed that while the resonant energies of the TOACs and RRICs of system shift towards the higher(lower) energies with the enhancement of η, V_1, they decrease with the augmentation of R. Thus, the findings of this study show that the optical properties of the structure can be adjusted by changing the magnitude of structure parameters and applied external fields.     

Exciton effects on the refractive index and optical absorption in wurtzite quantum wells via a fractional-dimensional space approach

The optical refractive index changes and absorption coefficients of quantum wells (QWs) are theoretically investigated with considering exciton effects within the framework of the fractional-dimensional space approach. The exciton wave functions and bound energies are obtained as a function of spatial dimensionality, and the dimension increases with the well width increasing. Then optical properties are obtained by using the compact-density matrix approach and an iterative method. Numerical results are presented for wurtzite ZnO/Mg_xZn_1−xO QWs. The calculated results show that the changes of refractive index and absorption coefficients are greatly enhanced due to the quantum confinement of exciton. And the smaller the QW width (dimension) is, the larger influence of exciton on the optical properties will be. Furthermore, the exciton effects make the resonant peaks move to a lower energy. In addition, the optical properties are related to the QW width, the incident optical intensity and carrier density.     

Excited state absorptions of an exciton bound to an ionized donor impurity in quantum dots

An investigation of an exciton bound in a parabolic two dimensional quantum dot by a donor impurity has been carried out by using the matrix diagonalization method and the compact density-matrix approach. The linear, third-order nonlinear, total optical absorption coefficients and refractive index changes have been calculated for the s-p, p-d, and d-f transitions. The results show that the parabolic potential has a great effect on the optical absorptions. The calculated results also reveal that as the angular momentum quantum numbers of transitions increase, the optical absorption and refractive index peaks shift towards lower energies and the absorption and refractive index intensities increase.     

Simultaneous Effects of External Electric Field and Conduction Band Nonparabolicity on Optical Properties of a GaAs Quantum Dot Embedded at the Center of a GaAlAs Nano-Wire

An investigation of the optical properties of a GaAs spherical quantum dot which is located at the center of a Ga1-xAlx As cylindrical nano-wire has been performed in the presence of an external electric field. The band nonparabolieity effect is also considered using the energy dependent effective mass approximation. The energy eigenvalues and corresponding wave functions are calculated by finite difference approximation and the reliability of calculated wave functions is checked by computing orthogonality. Using computed energy eigenvalues and wave functions, the linear, third-order nonlinear and total optical absorption coefficients and refractive index changes are examined in detail. It is found that （i） Presence of electric field causes both blue and red shifts in absorption spectrum; （ii） The absorption coefficients shift toward lower energies by taking into account the conduction band nonparabolicity; （iii） For large values of electric field the effect of conduction band nonparabolieity is less dominant and parabolic band is estimated correctly; （iv） In the presence of electric field and conduction band nonparabolicity the nonlinear term of absorption coefficient rapidly increases by increasing incident optical intensity. In other words, the saturation in optical spectrum occurs at lower incident optical intensities.     

Effects of electron–phonon interaction and impurity on optical properties of hexagonal-shaped quantum wires

We have investigated the influence of electron–phonon (e–p) interaction and hydrogenic donor impurity simultaneously on energy difference, binding energy, the linear, nonlinear and total refractive index changes and absorption coefficients of a hexagonal-shaped quantum wire. For this goal, we have used finite-element method (FEM), a compact density matrix approach and an iterative procedure. It is deduced that energy difference and binding energy decrease by changing the impurity position with and without e–p interaction. The dipole matrix elements have complex behaviours in the presence of impurity with and without e–p interaction. The refractive index changes and absorption coefficients increase and shift towards lower energies by enhancing a ₁ with central impurity. In the presence of central impurity, the absorption coefficients and refractive index changes enhance and shift toward higher energies when e–p interaction is considered.     

Refractive index change and absorption coefficient of T shaped quantum wires: comparing with experimental results

In this work, we have studied the optical properties of a GaAs/Al $_{x}$ Ga $_{1-x}$ As T-shaped quantum wire. In this regard, we have obtained the refractive index change and absorption coefficient. We have also studied the temperature effect on absorption coefficient. The results show that the refractive index change and absorption coefficient decrease and shift towards higher energies when temperature increases. Finally, to check our results, we have compared the absorption coefficient obtained from this work with experimental data at 5 K. Our theoretical prediction has some deviations with experimental data.     

Linear and Nonlinear Optical Properties of Spherical Quantum Dots:Effects of Hydrogenic Impurity and Conduction Band Non-Parabolicity

Simultaneous effects of an on-center hydrogenic impurity and band edge non-parabolicity on intersubband optical absorption coefficients and refractive index changes of a typical GaAs/Al x Ga 1 x As spherical quantum dot are theoretically investigated,using the Luttinger-Kohn effective mass equation.So,electronic structure and optical properties of the system are studied by means of the matrix diagonalization technique and compact density matrix approach,respectively.Finally,effects of an impurity,band edge non-parabolicity,incident light intensity and the dot size on the linear,the third-order nonlinear and the total optical absorption coefficients and refractive index changes are investigated.Our results indicate that,the magnitudes of these optical quantities increase and their peaks shift to higher energies as the influences of the impurity and the band edge non-parabolicity are considered.Moreover,incident light intensity and the dot size have considerable effects on the optical absorption coefficients and refractive index changes.     

Hydrostatic pressure and temperature effects of an exciton-donor complex in quantum dots

Using the matrix diagonalization method and the compact density-matrix approach, we studied the combined effects of hydrostatic pressure and temperature on the electronic and optical properties of an exciton-donor complex in a disc-shaped quantum dot. We have calculated the binding energy and the oscillator strength of the intersubband transition from the ground state into the first excited state as a function of the dot radius. Based on the computed energies and wave functions, the linear, third-order nonlinear and total optical absorption coefficients as well as the refractive index have been examined. We find that the ground state binding energy and the oscillator strength are strongly affected by the quantum dot radius, hydrostatic pressure and temperature. The results also show that the linear, third-order nonlinear and total absorption coefficients and refractive index changes strongly depend on temperature and hydrostatic pressure.     

Optical absorption and refractive index of a donor impurity in a three-dimensional quantum pseudodot

The optical absorption and refractive index of a donor impurity confined by a three-dimensional quantum pseudodot are studied using the matrix diagonalization method within the effective-mass approximation. The great advantage of our methodology is that it enables us to tune confinement strength and regime by varying two parameters in the model potential. Based on the computed energies and wave functions, the linear, third-order nonlinear and total optical absorption coefficients as well as the refractive index changes have been examined. The results are presented as a function of the incident photon energy for the different values of the chemical potential of the electron gas and the zero point of the pseudoharmonic potential. We find that the larger optical nonlinearity will be obtained by varying the zero point of the pseudoharmonic potential compared to the chemical potential of electron gas.     

Polaron effects on the optical absorption coefficients and refractive index changes in a square quantum well

The linear and nonlinear optical absorption coefficients and refractive index changes are obtained by using the compact density-matrix approach and an iterative procedure. With typical semiconducting GaAs materials, the linear, third-order nonlinear, total optical absorption coefficients and the optical refractive index have been examined. We find that the polaron effect has an important influence on the linear, third-order nonlinear, and total absorption coefficients as well as the refractive index changes.     

Morse势阱中线性和三阶非线性光折射率的改变

利用量子力学中的密度矩阵算符理论和迭代方法,导出莫尔斯（Morse）势阱中线性和三阶非线性光折射率改变的解析表达式,并以典型的GaAs／AlGaAs Morse势阱为例进行数值计算。数值结果表明,随着入射光强度增强,总的折射率改变将减少;随着势阱参数a的增大,总的折射率改变将减小;而随着载流子浓度的增加,总的折射率改变将增加。结果表明要获得较大的折射率改变,则需选取较小的入射光强度,较小的参数a,较大的载流子浓度,从而为实验研究提供理论依据。     

Optical absorption and refraction index change of a confined exciton in a spherical quantum dot nanostructure

Electronic energies of an exciton confined in a strained Zn_1−x Cd _x Se/ZnSe quantum dot have been computed as a function of dot radius with various Cd content. Calculations have been performed using Bessel function as an orthonormal basis for different confinement potentials of barrier height considering the internal electric field induced by the spontaneous and piezoelectric polarizations. The optical absorption coefficients and the refractive index changes between the ground state (L = 0) and the first excited state (L = 1) are investigated. It is found that the optical properties in the strained ZnCdSe/ZnSe quantum dot are strongly affected by the confinement potentials and the dot radii. The intensity of the total absorption spectra increases for the transition between higher levels. The obtained optical nonlinearity brings out the fact that it should be considered in calculating the optical properties in low dimensional semiconductors especially in quantum dots.     

Linear and nonlinear optical properties of strained GaN/AlN quantum dots: Effects of impurities,radii of QDs,and the incident optical intensity

The linear and nonlinear optical properties of cylinder GaN/AlN quantum dots with strain effects and impurity are investigated by taking into account the effects of the deformation potential and piezoelectric potential on the conduction band edge. The results are presented as a function of photon energies and QD radii. The optical absorption spectrum and refractive index changes have a blueshift in the presence of the impurity. With increasing distance of the impurity’s position along the growth direction, the peak values of the refractive index changes decrease and shift to higher photon energy. When the sizes of the QDs increase, redshift effects are observed and the relative amplitudes diminish. It can be found that the nonlinear effect becomes obvious with increase of the incident optical intensity. Then there is a “hole-burning” in the absorption coefficient spectra and two new peaks will appear in the total refractive index change spectrum when the optical intensity becomes larger enough. Finally it can be concluded that the intensity of the incident light and the position of the impurity play an important role in the linear and nonlinear optical properties.     

Study of optical properties in a cubic quantum dot

In this paper, the effect of hydrostatic pressure on both the intersubband optical absorption coefficients and the refractive index changes is studied for typical GaAs/Al _x  Ga_1?x As cubic quantum dot. We use analytical expressions for the linear and third-order nonlinear intersubband absorption coefficients and refractive index changes obtained by the compact-density matrix formalism. The linear, third-order nonlinear, and total intersubband absorption coefficients and refractive index changes are calculated at different pressures as a function of the photon energy with known values of box length (L), the incident optical intensity (I), and Al concentration (x). According to the results obtained from the present work, we have found that the pressure plays an important role in the intersubband optical absorption coefficient and refractive index changes in a cubic quantum dot.     

Nonlinear optical properties of an off-center donor in a quantum dot under applied magnetic field

The nonlinear optical properties of an off-center hydrogenic donor in a two-dimensional quantum dot under applied magnetic field are investigated in detail by using the matrix diagonalization method. Based on the computed energies and wave functions, the linear, third-order and total optical absorption coefficients as well as the refractive index changes have been examined between the ground state (L=0) and the first excited state (L=1). The results show that the ion position, the applied magnetic field, the confinement frequency, and the incident optical intensity have an important influence on the nonlinear optical properties of off-center donors.